Considering the near future, it’s safe to expect a “parity” of iOS+OS X vs. Windows within one or two years. The install base may remain larger for some time longer but the sales rate of alternatives will swamp it in due course.
The consequences are dire for Microsoft. The wiping out of any platform advantage around Windows will render it vulnerable to direct competition. This is not something it had to worry about before. Windows will have to compete not only for users, but for developer talent, investment by enterprises and the implicit goodwill it has had for more than a decade.
It will, most importantly, have a psychological effect. Realizing that Windows is not a hegemony will unleash market forces that nobody can predict.
Horace outlines why all of these things will happen, based on years of data going back to the 80's and current trend-lines in this article. Great analysis.